Bistable phototropic system

ABSTRACT

The invention relates to optical arrangements for storing binary information. The arrangements according to the present invention is a bistable optical storage system comprising at least two phototropic plates optically coupled with one another and arranged between two light sources, said sources being nonactinic in behavior vis-a-vis the nearest screen in each case, and being capable of causing the bleaching of the particular other screen, as the case may be. The plates may have either a variable transparency or a variable reflectivity. Writing in means are provided for bleaching at least one portion of said plates and erase means may be added for redarkening any portion the bleached area of said plates.

United States Patent Inventor Erich Spitz Paris, France Appl. No. 820,056

Filed Apr. 29, 1969 Patented Oct. 5, 1971 Assignee Thomson-CSF Paris, France Priority May 10, 1968 France BISTABLE PHOTOTROPIC SYSTEM 9 Claims, 8 Drawing Figs.

US. Cl 340/173 CC, 350/16 Int. Cl G1 1c 11/42, 61 1c 7/00 Field of Search 350/ 16 OP; 340/ 173 CC CONTROL INPUT Primary Examiner-Stanley M. Urynowicz, Jr. Attorney-Cushman, Darby, & Cushman ABSTRACT: The invention relates to optical arrangements for storing binary information. The arrangements according to the present invention is a bistable optical storage system comprising at least two phototropic plates optically coupled with one another and arranged between two light sources, said sources being nonactinic in behavior vis-a-vis the nearest screen in each case, and being capable of causing the bleaching of the particular other screen, as the case may be The plates may have either a variable transparency or a variable reflectivity. Writing in means are provided for bleaching at least one portion of said plates and erase means may be added for redarkening any portion the bleached area of said plates.

PATENTED 0m 5197! SHEET 3 UF 3 BISTABLE PHOTOTROPIC SYSTEM The present invention relates to optical memories which employ phototropic materials capable of storing optical signals in binary form. Phototropic materials include substances whose molecules, under the action of luminous radiation (in respect of which they are actinic), undergo transformations which bring about a reversible change in the mechanism of optical absorption.

The facility to erase signals recorded in a phototropic memory, arises from the reversibility of the change in state produced in the data carrier. However, this ready erasing takes place spontaneously to the detriment of the retention of the stored data, this because the phototropic material reverts more or less rapidly to its initial state after having been irradiated.

Since the data recorded in a memory should be capable of being retained for as long as may be desired, the phototropic materials employed are of the kind which undergo only a slow change while the information is stored. This condition is incompatible to a large extent, with the fact that the writing-in and the erasing which start and terminate the storing period, must be short. In order to reconcile such contradicting requirements, it is best to adopt a bistable system of operation, with rapid changeover from one stable state to the other.

According to the invention, there is provided a bistable optical storage arrangement forstoring binary information, said arrangement comprising: first and second phototropic plates, optically coupled with one another and capable of being bleached by respectively applying thereto first and second luminous radiations, said first and second radiations respectively having no bleaching action on said second and first plates; a first light source positioned for supplying said first radiation to said second plate; a second light source positioned for supplying said second radiation to said first plate and optical triggering means controlled by said information for causing said first source to bleach at least one portion of said first plate.

For a better understanding of the invention'and to show how the same may be carried into effect reference will be made to the drawing accompanying the ensuing description and in which: 7

' FIGS. 1 and 2 are explanatory diagrams;

FIG. 3 schematically illustrates a bistable system according to the invention, and its mode of operation;

FIG. 4 illustrates a modification of the system of FIG. 3;

FIG. 5 illustrates a further modification of the system of FIG. 3;

FIG. 6 illustrates a modification of the system of FIG. 4;

FIG. 7 illustrates another modifications;

FIG. 8 is a modification of the system of FIG.7.

In FIG. 1, a phototropic material 3 can be seen, illuminated by means of two light sources I and 2.

The molecules of the phototropic substance 3 can be excited by a light quantum emanating from the source I, this giving rise to a photochemical reaction bringing about a change in the optical absorption capability of said material. By way of example, a phototropic material constituted by a vitreous body upon which there have been deposited microcrystals of silver halide, will become opaque when irradiated by ultraviolet light emanating from the source 2; however, it regains its initial transparency under the influence of heat or when irradiated by visible or infrared light emanating from the source 1. In FIG. 2, there have been plotted respectively on the abscissae and the ordinates, the time and the optical absorption power a of a phototropic material. The curve A illustrates the law of variation of a when the material is irradiated by the source 2 between the times T, and T it will be seen that the material 3, initially transparent, rapidly goes opaque. Beyond the time T,, the coefficient or falls slowly since the source 2 is extinguished; the material gradually recovers its transparency over the section of the curve A following the point A; the return of the transparency or bleaching can be A" or curve A shows. The diagram of FIG. 2 illustrates that a phototropic material can be used as a binary memory element; the information is written into the memory between the times T, and T is retained between T, and T and is erased after T;,. However, a memory based on this principle has the drawback that the data retention time is very limited.

FIG. 3 shows at (a) a first example of a bistable optical system according to the invention; This system-is capable of retaining a binary information as long as may be required. The information is in the form of a light pulse.

The system of FIG. 3 (a) makes use of two phototropic materials X and Y and two light sources 1 and 4. The materials X and Y are of opaque kind and become transparent when irradiated respectively by the sources 1 and 4; the bleaching of the material X is produced only by the action of the light emitted by the source .1, Whilst that of the material Y is produced only by the action of the light emitted by the source 4. For the application envisaged, there is the choice of a number of phototropic materials which might be used; by way of example, there are those which undergo bleaching under the action of a suitable radiation, such as iron-thionine compounds, and those which, after having been rendered opaque by a suitable radiation, regain their transparency under the effect of another appropriate radiation; spiro-pyrane compounds belong to the latter category.

By fonning the screens 7 and 8 from the respective phototropic materials Y and X hereinbefore defined, and by directing the light produced by the sources I and 4, onto said screens through the medium of lenses 5 and 6, the bistable system of FIG. 3 (a) is produced.

FIG. 3 (b) illustrates as a function of the horizontal distance x, the variation in luminosity I in the system of FIG. 3 (a) when it is in one of the two stable states; FIG. 3 (0) relates to the other stable state in said system. It can be seen in FIG. 3 (b) that the light emanating from the lens 5 is heavily absorbed by the screen 7 which is initially opaque; the curve 9 in fact shows that the intensity of the light reaching through the screen 7, the screen 8, is low; the same applies to the curve 11 which shows that due to the opaque screen 8 the light that reaches the screen 7 has a very low intensity.

Thus both screen 7 and screen 8 become opaque and remain so since they do not receive the kind and quantity of light necessary to cause them to bleach. If, by means of a mirror I6 and an optical modulator 15 having a control input, a light pulse from source 4 is directed onto screen 7, said screen will cease to be opaque and will allow light from the source I to pass; this light, on reaching the screen 8, in turn renders the latter transparent. Under these circumstances, the curves 9 and II of FIG. 3 (b) respectively take the form of the curves l0 and 12' of FIG. 3 (c); the transparency of the screens is maintained in this condition since both are receiving light which is capable of maintaining them transparent.

When the bistable system has been tripped from the opaque state into the transparent state, tripping in the reverse direction can be produced, that is to say erasing, by cutting off one of the sources 1 or.4 or by interposing a shutter device between said sources.

FIG. 4 illustrates a bistable system equipped with an erasing device comprising an auxiliary phototropic screen 14 and a control light source 13; the other elements are the same as those illustrated in FIG. 3 (a) but the write-in device 1546 has not been shown. The screen I4 is initially transparent and remains so under the action of the light radiation emitted by the sources 1 and 4; it becomes opaque under the action of the light emitted by the source I3, the latter only being operated when it is desired to erase the information stored in the bistable optical system.

The bistable systems of FIG. 3 (a) and 4 are binary memory elements of a kind which can be assembled in sufficient number to store a plurality of optical signals. With this in view, the invention provides a phototropic plate comprising three layers 7, 8 and I4, and while multiple lenses or optical fibres are used to channel the light from the sources I and. 4 to distinct zones of the phototropic plate; each of these zones represents a memory element.

When the binary optical signals which are to be stored can be stored in the form of luminous spots undergoing a planar distribution, the phototropic screens 7 and 8 are placed according to the invention in the object and image planes of a stigmatic optical system. This arrangement is that shown in FIG. 5 and comprises the elements of FIG. 3 (a) with the exception of the lenses 5 and 6. A stigmatic optical system 18 is arranged between the screens 7 and 8 in order to form upon the screen 8 the image P of the point P on the screen 7; a further stigmatic system 20, associated with semitransparent mirrors 17 and 19, produces upon the screen 7 the image of an object 22 which constitutes the two-dimensional carrier of the optical data which are to be stored; the stigmatic system 20 also makes it possible to project the image of the screen 7 into the plane 21. The mirror 17 can be constituted by a transparent strip coated with a dichroic film or with dielectric films.

In the absence of any information, the screens 7 and 8 are opaque over the whole of their surface, provided that the sources 1 and 4 are not producing any optical bleaching upon the screens 7 and 8 respectively.

In order to write in a piece of information constituted by a small transparent zone P" on the object 22, the device 15 allows the light emitted by the source 4 to pass; this light illuminates the screen 7 at P, the latter screen becoming locally transparent and in turn allowing light from source 1 to pass; the light from source 1 illuminates the screen 8 at P and locally renders it transparent, allowing light from the source 4 to hit the screen 7 at P' and maintain it in transparent condition after the device 15 has ceased to transmit light. The write-in operation described above makes it possible to store a complete image since the state of transparency reached by the screen 7 is a replica of the state of transparency of the object 22; the stored image is projected into the plane 21 and can be erased, as has been shown hereinabove by interrupting the operation of one of the sources 1 to 4.

It goes without saying that in order to store the image, a step-by-step process can be used; the point-by-point recording can be effected by a moving light beam which is directed onto one of the screens 7 or 8; the condition which the light beam must satisfy is that of producing transparency in the screen upon which it falls; this condition can be satisfied either by using one of the light sources 1 or 4, or by using an auxiliary light source. Without departing from the scope of the invention, the object 22 may be substituted by an information source constituted by the screen of a cathode-ray tube, provided that the light produced by said screen is sufficient to produce bleaching of one of the screens 7 or 8.

It has been seen that in order to erase completely the information stored in screens 7 and 8 of FIG. 5, all that was necessary was to interrupt the operation of one of the light sources 1 or 4. Where it is desired only partially to erase the stored information, use is made of the device illustrated in FIG. 6.

The bistable system of FIG. 6 is designed for the storage of an image constituted by light spots whose brilliance can have either of two different values. It comprises the same elements as the system of FIG. 4, with the exception of the lenses 5 and 6; between the screens 7, 8 and 14 there are arranged stigmatic systems 18 and 23 which respectively produce the image of the screen 7 on the screen 8, and the image of the screen 8 on the screen 14. An erase light source 13 is associated with a lens and a semitransparent mirror 24 in order to illuminate any desired point N on the surface of the screen 14.

When the points P and P on the screens 7 and 8 are in the transparency state corresponding to the storage of an information element, the selective erasing of this information element is effected by applying the erase beam produced by the source 13, to the point N on the screen 14; the point N is the image point of the point P and it will be appreciated, by reference to the description of FIG. 4, that this point ceases to transmit light from the source 4 when illuminated by the source 13.

Under these circumstances the points P and P on the screens 7 and 8 become opaque again whilst the adjacent points are not affected by the erase function, since the screen 14 remains transparent around the point N.

Although FIGS. 5 and 6 separately illustrate the write-in and erase devices, it will be appreciated that these devices could be combined in one bistable optical system.

In the foregoing examples, phototropic screens operating by transmission have been considered, whereas in the examples which now follow the screens operate by reflection.

In FIG. 7, a bistable optical system can be seen which comprises three reflective phototropic screens 107, I08 and 114; a light source 101 illuminates the screen 114 through the medium of a lens 105; another light source 104 illuminates the screen 108 through the medium of a lens 106, and the screen 107 through the medium of a mirror 116 and a shutter a light source 113 likewise illuminates the phototropic screen 114 in order to render it absorbent during the erase phase. The phototropic screens 107 and 108 are initially absorbent and do not undergo any bleaching when respectively illuminated by the sources 101 and 104; on the other hand, the screens 107 and 108 become reflective when respectively illuminated by the sources 104 and 101. As long as the source 113 does not illuminate the screen 114, the light from source 101 is directed onto the screen 107 and is not transmitted to the screen 108; also, the screen 108 does not transmit to the screen 107 the light which it receives from the source 104, so that the screens 107 and 108 do not exhibit any bleaching. In contrast, if the screen 107 is illuminated by a light pulse emanating from the source 104 and passing through the devices 115 and 116, it becomes reflective and enables the light from source 101 to reach the screen 108; the latter then likewise becomes reflective and transmits the light from the source 104 to the screen 107. When the screens 107 and 108 are in reflective state, they can be placed in the nonreflective state by illuminating the screen 114 by means of the source 113; under the action of this source, the screen 114 ceases temporarily to reflect the light from the source 101, thus returning the system to the initial condition.

In FIG. 8, a bistable optical system can be seen comprising the same elements as shown in FIG. 7, with the exception of the elements 114 and 113 which are used for information erasing. The screens 107 and 108 are optically coupled by a stigmatic system 118 which produces the image of screen 107 on screen 108. A stigmatic system 120 enables the image of an object 122 to be reproduced upon the screen 107. This object carries the information which is to be stored. Initially, the screens 107 and 108 are absorbent over their whole area; when the shutter 115 allows light to pass from the source 104, the mirror 119 directs it onto the object 122; the transparent point P" on the object 122 illuminates the screen 107 at P; this localized illumination enables the screen 107 to become reflective at P for the light coming from source 101; the screen 108 is thus illuminated at P and in turn becomes locally reflective. The information element thus stored can be read out by a stigmatic system 117 which reproduces the image of screen 108 in the readout plane 121.

As in the foregoing examples, general erasing can be carried out by interrupting the operation of one of the sources 101 or 104; it can also be carried out selectively by using an auxiliary phototropic screen.

Without departing from the scope of the invention, the optical coupling between the phototropic screens can be achieved by dioptric or catoptric stigmatic systems and bunches of optiinformation, said arrangement comprising: first and secondphototropic plates, optically coupled with one another and capable of being bleached by respectively applying thereto first and second luminous radiations, said first and second radiations respectively having no bleaching action on said second and first plates; a first light source positioned for supplying said first radiation to said second plate; a second light source positioned for supplying said second radiation to said first plate and optical triggering means controlled by said information for causing said first source to bleach at least one portion of said first plate.

2. A storage arrangement as claimed in claim 1, further comprising optical means for erasing the information stored therein, said erasing means comprising a third phototropic plate intersecting the path of the radiation supplied by one of said sources and capable of being darkened by applying thereto a third luminous radiation, and a third light source for supplying said third radiation to at least one selected portion of said third plate.

3. A storage arrangement as claimed in claim 1, further comprising stigmatic optical means located between said plates for forming on one of said plates an image of said other plate.

4. A storage arrangement as claimed in claim 1, wherein said plates are variable transparency plates.

5. A storage arrangement as claimed in claim 1, wherein said plates are variable reflectivity plates.

6. A storage arrangement as claimed in claim 1, wherein said optical triggering means comprise an optical modulator having a first input for receiving said information, a further input for receiving a portion of the luminous energy supplied by said first source and an output supplying a writing beam directed on said first plate.

7. A storage arrangement as claimed in claim 3, wherein said triggering means comprise optical switching means having an input for receiving a portion of the luminous energy supplied by said first source and an output for illuminating a transparency carrying said information, and further optical stigmatic means positioned between said transparency and said first plate for projecting thereon an image of said transparency.

8. A storage arrangement as claimed in claim 7, further comprising optical means for erasing at least one portion of the information stored within said first plate.

9. A storage arrangement as claimed in claim 8, wherein said erasing means comprise a third phototropic plate inserted on the path of the radiation supplied by said first source to said second plate and capable of being darkened by applying thereto a third luminous radiation, a third light source for feeding said third radiation to at least one selected portion of said third plate and further optical stigmatic means inserted between said third plate and said second plate for forming thereon an image of said third plate. 

1. A bistable optical storage arrangement for storing binary information, said arrangement comprising: first and second phototropic plates, optically coupled with one another and capable of being bleached by respectively applying thereto first and second luminous radiations, said first and second radiations respectively having no bleaching action on said second and first plates; a first light source positioned for supplying said first radiation to said second plate; a second light source positioned for supplying said second radiation to said first plate and optical triggering means controlled by said information for causing said first source to bleach at least one portion of said first plate.
 2. A storage arrangement as claimed in claim 1, further comprising optical means for erasing the information stored therein, said erasing means comprising a third phototropic plate intersecting the path of the radiation supplied by one of said sources and capable of being darkened by applying thereto a third luminous radiation, and a third light source for supplying said third radiation to at least one selected portion of said third plate.
 3. A storage arrangement as claimed in claim 1, further comprising stigmatic optical means located between said plates for forming on one of said plates an image of said other plate.
 4. A storage arrangement as claimed in claim 1, wherein said plates are variable transparency plates.
 5. A storage arrangement as claimed in claim 1, wherein said plates are variable reflectivity plates.
 6. A storage arrangement as claimed in claim 1, wherein said opticaL triggering means comprise an optical modulator having a first input for receiving said information, a further input for receiving a portion of the luminous energy supplied by said first source and an output supplying a writing beam directed on said first plate.
 7. A storage arrangement as claimed in claim 3, wherein said triggering means comprise optical switching means having an input for receiving a portion of the luminous energy supplied by said first source and an output for illuminating a transparency carrying said information, and further optical stigmatic means positioned between said transparency and said first plate for projecting thereon an image of said transparency.
 8. A storage arrangement as claimed in claim 7, further comprising optical means for erasing at least one portion of the information stored within said first plate.
 9. A storage arrangement as claimed in claim 8, wherein said erasing means comprise a third phototropic plate inserted on the path of the radiation supplied by said first source to said second plate and capable of being darkened by applying thereto a third luminous radiation, a third light source for feeding said third radiation to at least one selected portion of said third plate and further optical stigmatic means inserted between said third plate and said second plate for forming thereon an image of said third plate. 